Although industrial noise pollution has always existed, it has become more acute through the use of larger and higher speed machinery needed to increase production output. Also, modern jet engines, as is well-known, produce a higher perceived noise level than the reciprocating internal combustion engines which they have generally replaced. To diminish this noise level, the inlet and exhaust ducts of jet engines commonly contain sound absorptive linings of the laminar absorber types.
One of the types of air-borne broad-band prior art sound absorbing panels is a honeycomb panel comprising an impermeable backing sheet, a permeable flow resistive facing cover and a honeycomb core therebetween, wherein the cells of the honeycomb core are configured so that adjacent cell subcompartments within each honeycomb cell have different resonant frequencies. One such honeycomb panel type absorber is disclosed in U.S. Pat. No. 3,831,710 entitled "Sound Absorbing Panel" by Leslie S. Wirt which is incorporated herein by reference and which is assigned to the assignee of the present invention. However, two problems arose in the manufacture of this sound absorbing panel: the geometry of the core required expensive and precise fine tooling; and since, in many applications, such panels were exposed to liquids, means had to be provided to drain the fluids which further complicated the design of the core.
It is therefore an object of the present invention to provide a novel and improved sound absorbing panel.
A further object of the present invention is to provide an improved sound absorbing panel that is easy to manufacture.
Another object of the invention is to provide an improved sound absorbing panel which exhibits broad-band acoustical absorption, and which is suitable for use under extreme environmental conditions and, in particular, exposure to fluids.